1. Field of the Invention
At least one aspect of the present invention relates to at least one of a projection optical system and an image projecting apparatus.
2. Description of the Related Art
A conventional technique related to at least one of a projection optical system and an image projecting apparatus is disclosed.
For example, Japanese Patent Application Publication No. 2007-079524 (patent document 1) discloses a projection optical system for conducting enlargement and projection from a primary image plane at a reduction side to a secondary image plane at an enlargement side, characterized in that the projection optical system includes a first optical system for imaging an intermediate image of the primary image plane and a second optical system having a concave reflection surface for forming the secondary image plane from the intermediate image, wherein a light ray traveling from a center of the primary image plane to a center of the secondary image plane intersects an optical axis of the first optical system, then is reflected from the concave reflection surface, again intersects the optical axis, and arrives at the secondary image plane.
For example, Japanese Patent Application No. 2008-116688 (patent document 2) discloses a projection optical system for conducting enlargement and projection from a primary image plane at a reduction side to a secondary image plane at an enlargement side, characterized in that the projection optical system includes a first optical system for imaging an intermediate image of the primary image plane, which is composed of a first group having a negative refractive power, a second group having a positive refractive power, an intervening stop, and a third group having a positive refractive power, from an intermediate image side of the stop, and a second optical system having one concave reflection surface formed at a secondary image side of the intermediate image, wherein each surface composing the first optical system and the second optical system is configured as a rotationally symmetric surface centered at a common optical axis, wherein a light ray traveling from a center of the primary image plane to a center of the secondary image plane intersects the optical axis, then is reflected from the concave reflection surface, again intersects the optical axis, and arrives at the secondary image plane, and wherein the following condition formulas:0.5<φ1/φ2<3  (1)1<AST/ASS<5  (2)|AST|/L12<1  (3)are satisfied and the following condition formula:−3<Krel  (4)is satisfied for all of lenses composing the first optical system and having a positive refractive power (herein,
φ1: a refractive power of the first optical system
φ2: a refractive power of the second optical system
|AST|: a position of intermediate imaging on a meridional plane of the first optical system
|ASS|: a position of intermediate imaging on a sagittal plane of the first optical system
L12: a distance on the optical axis between the first optical system and the second optical system
Krel: a temperature coefficient of a refractive index).
For example, Japanese Patent Application Publication No. 2008-165187 (patent document 3) discloses a projection optical system including a first optical system for forming a first image conjugate to an object and a second optical system for projecting a second image conjugate to the first image onto a surface to be projected on, wherein at least one of the first optical system and the second optical system includes at least one optical element movable with respect to the object, and wherein the projection optical system is characterized in that the at least one optical element with respect to the object is moved whereby an image distance for the projection optical system is changed and a size of the second image is changed.
However, it has not been easy for even a person skilled in the art related to a projection optical system or an image projecting apparatus to provide a more compact projection optical system capable of projecting a better image onto a surface to be projected on at a shorter distance or a more compact image projecting apparatus capable of projecting a better image onto a surface to be projected on at a shorter distance.